This invention relates generally to presses of the continuous type such as are employed for extracting water or other liquids for various fluids or semi-fluid feed materials, and specifically to cone presses wherein the screening surfaces are shaped to conform to truncated cones.
Although the invention will be described in terms of extracting water from paper pulps or slurries, the invention is also applicable in extracting liquid from said other pulps, slurries, and semi-solid materials. Examples of other materials include sewage sludges, vegetable and fruit pulps, tomato and grape pomace, citrus peel, fish or fish process slurries, cannery wastes, and brewers and distillers grains.
In extracting liquids from feed materials in a press, filter screens are caused to compress the solids of the feed to effect a reduction in volume, which in turn causes liquid to be expelled through the screen. In a cone press, a pair of rotating wheels or disks are faced with screens shaped to conform to truncated cones, and the volumetric compression is done by the co-acting cones.
The construction of the cone press is such that the axes of rotation of the cone wheels are normally intersecting but out of alignment whereby the screens are brought into close opposition in a pinch or nip region, but are separated apart a substantial distance in a wide region diametrically opposite the pinch region. The two cone wheels are driven continuously in the same direction, usually by gear pinions on a common drive shaft which mesh with gear teeth provided on the peripheries of the wheels or by a chain drive which also operates on teeth on the peripheries of the wheels. A casing extends between the peripheries of the wheels and is provided with an inlet for the feed slurry and also with an outlet for the removal of the squeezed or dewatered solids. A partition member or plow extends between the cone wheels from one portion of the casing to the center area between the cones and serves to direct the flow of material between the screen surfaces.
In operation, the feed is continuously introduced in the wide region of the inlet and continuously carried between the screens through the pinch or nip region. The squeezed or dewatered solids are withdrawn at the outlet shortly after passing through the nip region, and the liquid is caused to be expressed through the screens, and is collected by an appropriate means. In one form of early prior such continuous press, the location of the wheels remained fixed during operation, so that the spacing of the screens in the pinch region remained constant.
Continuous presses of the prior type described above are subject to a number of disadvantages. The performance and capacity of such a press is dependent upon the rate of feed and also upon variations in the solids content of the feed material. This is because the pressure applied to the solids in the pinch region varies with the amount of solids present, and this in turn is dependent upon the rate of feed and the solids content of the feed. At high feed rates, overloading of the press can occur. In many instances, this has the effect of limiting the overall capacity of the machine, and it also interferes with attainment of a desired residual liquid content in the expressed solids and in the maintenance of a desired residual liquid content over long operating periods.
Another disadvantage of the early prior cone presses is that the means generally employed to take the thrust against the core wheels have been thrust rollers tracking on the outer faces of the wheels, or bearings into which the cone wheel support shaft is journalled, and these are generally unsatisfactory because they are complicated and subject to excessive wear. Further, such prior continuous presses have not been amenable to quick adjustments to accommodate different types of feed materials, and their design has not facilitated ready access to the screens for replacement or repair, or for cleaning.
More recent prior cone presses wherein the cone wheel supports are pivotally connected with respect to a fixed point will allow adjustment to maintain a constant pressing force. They do so by adjusting mainly the nip width, which is the distance between the conical screen surfaces at the point of their closest proximity. As a result, the compression ratio, which is the ratio between the large feed flow area and the small nip flow area, also changes. However, the distribution of nip widths and of the compression ratios along the radius of the cone wheels changes in a preset, i.e., non-adjustable, manner. Typical such presses are shown in U.S. Pat. Nos. 3,447,450 (Wilhelm) and, 3,105,434 (Messing).
A cone press according to U.S. Pat. No. 3,447,450 (Wilhelm), which features support structures for the wheels with pivotal supports on one end of the support structures and a yieldable connection at the other end of the support structures, exhibits a complicated relationship between compression ratio and nip width. As an example, when the side structures are moved apart, the nip width changes along the radius of the wheel, namely, it becomes greater for greater radii. The compression ratio will also change, namely it generally decreases, but the rate of decrease is greater at greater radii. As a result, the residual moisture content of material at the greater radii would be greater than the moisture content of the material in the center areas. Thus, when more material is fed in the press, the discharge in the outer regions -- of greater radii -- will become relatively wet.
In order to overcome the problems caused by using thrust rollers, some presses have employed a friction pad, such as a high density polyethylene or nylon annulus. Such a thrust bearing structure is relatively simple in construction and allows for a uniform distribution of the thrust generated in the pressing operation. But such structures require a high power input in order to overcome the friction between the pads and the rotating disk.
Yet an additional problem with prior press structures is that generally no provision was made to prevent moisture from leaking past the tapered barrier or plow structure and rewetting the dewatered solids when the press was used to dewater a low solids content slurry. Presses have employed rubber strips between the rotating assemblies and the stationary frame, but they did not seal the center or hub area. Sealing is a further problem in presses wherein the cone wheels are pivotally connected.
Thus a need exists for an improved press which has a more uniform dewatering capability, an improved thrust bearing means, and an improved seal means associated with the barrier or plow structure.